1. Field of the Invention
This invention relates to an aspherical reflection preventive film containing therein an aspherical thin film layer.
2. Description of the Prior Art
Manufacture of an aspherical lens by abrasion requires a great deal of time and labor, hence a high manufacturing cost inevitably reflects on mass-produced articles such as photographic lenses of a standard focal length. In order to avoid such high manufacturing cost, there have so far been attempted various methods such as a method of varying the refractive index by ion injection, a method of etching the abrasion surface with ion beam, and others. Of these methods, the manufacture of the aspherical film by evaporative deposition provides sufficient possibility to mass-production of the article, if the thickness of the aspherical film does not exceed 10 microns, or so.
It is, however, difficult to manufacture, by the evaporative deposition method, the aspherical film coating having the same refractive index as a lens having a refractive index of from 1.45 to 1.82 or so, since the kind of material used for the evaporative deposition is limited. Accordingly, when the refractive index of the lens is close to that of the aspherical film, e.g., as a lens BK-7 having a refractive index of 1.52 is close to an SiO.sub.2 film having a refractive index of 1.49, there is no problem at all. However, when the refractive index of the lens differs from that of the aspherical film, there takes place such a situation that, no effect can be derived from provision of the reflection preventive film on the aspherical film, since reflection occurs at the interface between the lens and the aspherical film coating. As the consequence, when the aspherical thin film is directly provided on the substrate and a group of thin film layers are further provided on it for the purpose of reflection prevention, the residual reflective power is large, so that the reflection prevention characteristic of the thin film layers including the aspherical thin film could not be said to be satisfactory.